# Crawling and Crazing: What Went Wrong

Unit: The Final Bake: Glaze Firing
Topic: Common Glaze Faults
URL: https://claybook.studio/learn/crawling-and-crazing-what-went-wrong/

# Reading Glaze Defects Like a Detective

Glaze defects are frustrating, but every defect has a cause, and once you know the cause, you can prevent it. The three most common defects in studio pottery are crawling, crazing, and pinholes. This lesson covers crawling and crazing.

## Crawling

Crawling is when the glaze pulls back from the clay surface during firing, leaving bare patches of unglazed clay surrounded by thick glaze edges. It looks like the glaze ran away from parts of the pot.

**Causes:**
*   Glaze applied over a dusty or oily surface. The glaze has nothing to grip.
*   Glaze applied too thickly. The layer cracks as it dries, and those cracks widen during firing.
*   Glaze applied over an under-fired bisque with too many impurities in the clay.

**Fixes:**
*   Wipe the pot down with a clean, barely damp sponge before glazing.
*   Apply glaze in thinner, even layers rather than one thick coat.
*   Make sure your bisque firing is complete before glazing.

## Crazing

Crazing is a fine network of cracks in the glaze surface, like a shattered windshield. The glaze has cracked after firing and cooling.

**Causes:**
*   The glaze and clay body have different **thermal expansion coefficients**: they expand and contract at different rates as the kiln heats and cools.
*   If the glaze contracts more than the clay during cooling, it shatters into a craze pattern.

**Fixes:**
*   Use a glaze formulated for your specific clay body.
*   A slow, controlled cool-down can sometimes reduce crazing.
*   If a glaze consistently crazes on your clay, it is not the right fit; switch glazes.

## Defect Diagnosis Flow
When something looks wrong:
* Bare islands with thick glaze edges → crawling
* Fine crack network only in glaze → crazing
* Tiny craters with raw clay showing → pinholes

Naming the defect correctly is the first step to fixing it.

## Dig Deeper

Both crawling and crazing are well-documented [glaze defects](https://en.wikipedia.org/wiki/Glaze_defects) with specific causes rooted in chemistry and physics. Crazing in particular is driven by mismatched [thermal expansion](https://en.wikipedia.org/wiki/Thermal_expansion) rates between the glaze and the clay body, a concept that applies to all ceramic materials from [earthenware](https://en.wikipedia.org/wiki/Earthenware) to porcelain.

## Common Mistakes and Quick Fixes

*   **Guessing the defect name**: Identify pattern first, then choose correction.
*   **Applying thicker glaze to “cover” defects**: Thickness often worsens crawling and running.
*   **Skipping surface prep**: Dust and oils are major adhesion failures.
*   **Trying one fix once and stopping**: Defect control requires staged testing over multiple firings.

## Practice Exercise

Make a defect board with small tiles labelled crawling, crazing, and pinholes using intentional test conditions. Keep it near your kiln as a visual diagnosis reference for future loads.

## Check your understanding

### Question 1: What is the most likely cause of glaze crawling?

- [ ] A. The kiln temperature was too low
- [x] B. Glaze applied over a dusty surface or applied too thickly
- [ ] C. The glaze was too thin
- [ ] D. The pot was not centred on the shelf

Tip: Crawling is usually caused by glaze applied over a dusty or oily surface, or applied too thickly so it cracks while drying. The glaze loses adhesion and pulls back during firing.

### Question 2: What causes crazing in a glaze?

- [ ] A. The glaze was applied too thinly
- [x] B. The glaze and clay body have mismatched thermal expansion rates
- [ ] C. The kiln was fired too slowly
- [ ] D. The clay was not bone dry before bisque firing

Tip: Crazing happens when the glaze and clay body have different thermal expansion rates. The glaze contracts more than the clay during cooling, causing it to crack in a fine network pattern.